Traditionally, the stack is a FILO (first in, last out) allocation area. Generally they start at the top of memory and work their way down as space is allocated, then back up as it is freed.

The heap might start at the low end of unused memory (above the program and libraries) and is used for allocating space with arbitrary release order. This means it has to keep track of what is used and what is available as well as when to "grow the heap" by allocating more pages of memory.

Garbage collection is one way of managing a heap.